This is a trick question. Where would you expect to find the greatest variety of birds?

Downtown, in a city?

Or far, far from downtown — in the fields, forests, mountains, where people are scarce?

Or in the suburbs? In backyards, lawns, parking lots and playing fields?

Not the city, right?

"Everything I have learned as a conservation biologist tells me cities are bad for biodiversity," writes John Marzluff, of the University of Washington.

We all know this. Anyone who goes to downtown Chicago, Toronto, Seattle, LA, Boston or New York will see the same five birds over and over: sparrows, starlings, mallards (ducks), geese and, of course, street pigeons. Same goes for downtowns in Europe, Asia and South America. These five bird types are always there, always the same, never surprising. Rather than yawn, scientists have a category for this: "biotically homogenous." We've made cities. They've moved in.

A Seattle Experiment

But now comes a surprise. Actually, several surprises. When Marzluff and his students went to downtown Seattle to count bird species, within the first 10 to 15 minutes they spotted pigeons, finches, sparrows, crows and an occasional hummingbird. Their count was 10 to 15 different kinds of birds — not many, but they expected that.

When they went the other way (to the far edge of the metropolitan area near the Cascade Mountains, where there is mostly forest, protected parks, reservoirs, and humans are sparse), in the first 10 to 15 minutes, they found a very different set of birds (woodpeckers, wrens, warblers, chickadees). In all, 20 different species — more, but not many more than downtown.

Then they went to the in-between zone, the Seattle suburbs, where they expected an in-between count, something like 12 different kinds of birds. But that's not what happened.

"We were astonished," Marzluff writes. The suburban count (again in the first 10 minutes) was "30 or more species," says Marzluff, some from downtown, some from the mountains, but also spectacularly new samples of "violet-green swallows, willow flycatchers, killdeer, orange crowned warblers, American goldfinches, and Bewick's wrens ... [plus a few] white crowned sparrows." The suburbs produced, by far, the most biologically diverse collection of birds.

'Subirdia'

What? This region that's all sprawl, a hodgepodge of strip malls, yards, highways, parking lots, hedges, fences, is "a mecca for birds"? More than a forest? No way, thought Marzluff. So he counted again. Then again. And after checking and compiling "more than 100 locations in and around Seattle," he writes, he and his team discovered "a consistent, but unexpected relationship between the intensity of development and bird diversity."

To his great surprise, Marzluff concluded that the "greatest diversity was not in the most forested setting. Instead, bird diversity rose quickly from the city center to the suburbs and then dropped again in the extensive forest that eases Seattle into the high Cascades."

If you plotted it on a graph, bird biodiversity looks like this ...

He had just discovered, he writes, "subirdia." And that's the name of his forthcoming book, due this fall, called Welcome to Subirdia.

But Why?

So what have suburbs got that forests don't? Suburbs, he says, offer a wide range of artificially designed garden habitats, providing a smorgasbord of nuts, fruits, seeds, insects and ponds, in dense concentrations. Because they are rich with different kinds of bird food, suburbs are rich with different kinds of birds.

In Leicester, England, one survey found 422 different plant species in a single garden. Another census of 61 private yards in Britain found 1,166 vascular plants, 80 different lichens, 68 varieties of moss.

But let's not get crazy about this: Suburbs are not the birdiest zones on Earth. Any patch of tropical forest, with its dazzling populations of plant and animal life, will trump a garden-rich suburb. But if you are comparing suburban bird diversity with temperate wild spaces — say the Cascades, the Smokies or the Adirondacks — the suburbs, shockingly, win.

Birds And 'Burbs

And not just in Seattle. Marzluff writes that "throughout Britain, in deciduous woodlands of California and Ohio, grasslands of Arizona, forests of Japan, and shrublands of Australia, moderate levels of urbanization also provide an abundance of various resources that increases the number of bird species beyond that found in either wilder, or more densely populated settings."

So, like 40 percent of America's humans, a big hunk of America's bird species have chosen to live the suburban life. It's a bird boom, I'm surprised to say, I had never noticed.

In his forthcoming book, Welcome to Subirdia(to be released in September), Marzluff drops one last bomb — "a real stunner," he says. Every March, he spends a few days in Yellowstone National Park, up on the northern edge. That's a 2.2 million-acre expanse of wild space — very, very big. In 2013, he counted 26 bird species there.

Then he got on a plane and headed east to New York City, and found himself along Sixth Avenue, where he saw the usual "house sparrows, European starlings, and rock pigeons." And when he reached Central Park, he entered and found some mallards and the geese, so the usual urban bird quintet was accounted for; but as he walked around, he spotted a cardinal, then a blue jay, then a white-throated sparrow, then a black-crested titmouse, then a Cooper's hawk, then some crows, some blackbirds, three varieties of woodpeckers, wood ducks, cormorants, red-tailed hawks, herons, mourning doves — altogether 31 species. He saw a greater variety of birds in Central Park than he did in Yellowstone. We all know New York attracts exotic people, but birds too? Wild and wide open spaces, apparently, offer no special advantages. "From a bird's perspective," Marzluff writes, "large park[s] created by human hands or by nature are not all that different." Huh.